1. Field of the Invention
This invention relates to methods and apparatus for measuring the diameter of optical waveguide fibers. More particularly, the invention relates to 1) a technique for measuring fiber diameter which is insensitive to fiber ellipticity and 2) a technique for characterizing the extent to which a fibers cross-section is non-circular.
2. Description of the Prior Art
The precise measurement of the outside diameter of optical waveguide fibers is of central importance in both the manufacturing and quality control of such fibers. Among other things, diameter measurements are used to control the fiber drawing process and to select fiber suitable for commercial use.
U.S. Pat. Nos. 3,982,816 and 4,067,651 to Lawrence Watkins disclose an optical technique for measuring fiber diameter which is widely used in the industry. The basic components of the Watkins system are schematically illustrated in FIG. 1. As shown therein, optical waveguide fiber 13, whose cross-section has been greatly expanded for purposes of illustration, is transversely illuminated by light 15 of sufficient spatial coherence and monochromaticity to create a discernible interference pattern in the far field, that interference pattern being created by the superposition of light reflected from the fiber surface 17 and light refracted through the fiber body 13. In practice, a laser, e.g., a HeNe laser, is the preferred light source because of its wavelength stability. The following discussion is thus in terms of a laser light source, it being understood that other light sources having sufficient spatial coherence and monochromaticity can be used if desired.
As explained in the Watkins patents, in the far field, this reflected and refracted light interferes to form fringe pattern 19. For an optical waveguide fiber having a core and a cladding, the fringe pattern will in general be a function of the wavelength of the incident light and the indices of refraction and the diameters of both the core and the cladding. However, as shown by Watkins, if the core/clad ratio is not too large and if the fringe pattern is examined at sufficiently large angles, e.g., above about .+-.50.degree. in FIG. 1 for core/clad ratios of less than about 0.5, the pattern will depend almost exclusively on the diameter and index of refraction of the cladding.
Accordingly, if the index of refraction (n) of the cladding is known, the outside diameter (d) of the fiber can be determined by analyzing the fringe pattern. Specifically, the diameter can be approximated with good precision by counting the number of full and partial fringes (N) between two angles (.theta..sub.a and .theta..sub.b) and then using the following equations to calculate d: ##EQU1## where .lambda. is the wavelength of the laser light used to illuminate the fiber. Note that in equation 3 there is a direct relationship between diameter and fringe count. In practice, given an invariant clad index and an invariant wavelength, one can calibrate the system with an empirical constant, which when multiplied by the number of fringes, gives the diameter.
Refinements of the basic Watkins technique can be found in various patents and publications including Frazee, Jr. et al. U.S. Pat. No. 4,027,977 (determination of core/clad ratio by detecting the angle of maximum modulation of the fringe pattern); Murphy et al. U.S. Pat. No. 4,280,827 (use of delay circuits and comparators to analyze fringe patterns); Smithgall, Sr. U.S. Pat. No. 4,046,536 (analysis of fringe counts in the presence of "dropouts" resulting from faults in the fiber); and Mustafa Abushagur and Nicholas George, "Measurement of Optical Fiber Diameter Using the Fast Fourier Transform," Applied Optics, Jun. 15, 1980, vol. 19, no. 12, 2031-2033 (use of fast Fourier transforms to analyze fringe patterns).
Other optical techniques for measuring fiber properties, including fiber diameters, can be found in Bailey et al. U.S. Pat. No. 4,924,087 (detection of fiber defects using light scattered out of the plane of the basic diffraction pattern); Douklias U.S. Pat. No. 4,501,492 (detection of fiber defects and testing of fiber diameters using a spatial filter prepared using diffracted/scattered light from a defect-free fiber); Eichenbaum U.S. Pat. No. 4,363,827 (detection of "caustic" surfaces in the pattern of scattered light produced by a coated optical fiber in order to control the coating process); Maillard et al. U.S. Pat. No. 4,541,856 (use of "diffused" light to detect bubbles, blisters, and solid particles in a stream of molten glass); Millet et al. U.S. Pat. No. 4,847,509 (use of two perpendicular optical systems to measure fiber diameter in which each system forms a blurred image of the fiber on a strip of photodetectors); Preshy U.S. Pat. No. 4,307,296 (measurement of core diameter by inducing fluorescence of an index-modifying dopant in the core); Young, II U.S. Pat. No. 4,136,961 (detection of defects in glass blanks by rotating the blank through a thin beam of light); and A. Ashkin, J. M. Dziedzic, and R. H. Stolen, "Outer Diameter Measurement of Low Birefringence Optical Fibers by a New Resonant Backscatter Technique," Applied Optics, Jul. 1, 1981, vol. 20, no. 13, 2299-2303 (use of near-field resonant backscattered light to determine fiber diameters and ellipticity).
Equations 1-3 are based on the assumption that the fiber is circular. In practice, fibers are not perfectly round but tend to have at least some ovoid or elliptical characteristics. This non-circularity can cause over or under estimates of the average fiber diameter by as much as one percent. These errors in diameter measurement limit the level of process control and product uniformity which can be achieved. In particular, one needs to hold the average fiber diameter at a particular value during the drawing of optical waveguide fibers from preforms. Errors of one percent in the diameter of optical waveguide fibers are considered to be large errors relative to the level of uniformity expected for such fibers.
Accordingly, there is need for a fiber diameter measurement technique which is insensitive to ellipticity, i.e., a technique which will determine the average diameter of a non-circular fiber with high precision. Moreover, there is also a need for a method which can characterize the ellipticity of a fiber so that manufacturing techniques, such as fiber draw, can be modified and/or controlled to minimize non-circularity.
The Watkins patents include a proposed technique for measuring the degree of a fiber's non-circularity. According to Watkins, non-circularity results in a spatial shift of the interference fringe patterns, with the pattern for angles greater than 0.degree. moving, for example, closer to 0.degree., and the pattern for angles less than 0.degree. moving away from 0.degree..
The Watkins patents propose using these shifts to ascertain the non-circularity of fibers by comparison of measured shifts with those for calibrated fibers. See column 9, line 66, to column 10, line 21, of U.S. Pat. No. 3,982,816 and column 9, lines 35-58, of U.S. Pat. No. 4,067,651. Watkins also describes performing fringe counts on each side of the fiber, and in claim 7 of U.S. Pat. No. 4,067,651, Watkins claims such a process wherein the difference between the fringe counts is determined and compared with fringe count differences for calibrated fibers.
Significantly, although Watkins discusses the problem of elliptical fibers, he does not provide a method for measuring fiber diameter which is insensitive to ellipticity. As to his proposed technique for estimating the degree of non-circularity, that technique suffers from a number of problems. First, it requires calibration with fibers of known ellipticity, presumably having a wide range of major to minor axis ratios. Such fibers would in general be difficult to produce, and the subsequent calibration of the system using a series of such fibers would clearly be tedious and time consuming.
Second, in accordance with the present invention, it has been determined that the detection of fiber ellipticity is subject to "blind spots" depending on the orientation of the fiber relative to the light source (see, for example, the 0.degree., 90.degree., and 180.degree. points in FIG. 6). Watkins does not in any sense address this problem.
Third, in accordance with the present invention, it has been determined that the fringe pattern produced by a non-circular fiber is a function of the orientation of the fiber's major and minor axes with respect to the direction of the light source (see, for example, FIG. 3). Accordingly, for a meaningful comparison to be made in the Watkins system between an unknown fiber and a calibration fiber, both fibers would have to have the same orientation with respect to the laser beam. Plainly, this would be difficult to do for stationary fibers. For a moving fiber, such as a fiber being drawn from a preform, the orientation of the fiber's non-circularity is generally unknown, thus making the Watkins system unworkable.